KR20130142056A - Femtosecond laser apparatus and femtosecond laser system including the same - Google Patents

Abstract

Provided are a femtosecond laser apparatus and a femtosecond laser system including the same. The femtosecond laser apparatus includes a first laser medium having an Ng axis, an Np axis and an Nm axis spatially vertical thereto; a second laser medium having an Np axis substantially in parallel with the Ng axis of the first laser medium, an Nm axis substantially in parallel with the Np axis of the first laser medium, and an Ng axis substantially in parallel with the Nm axis of the first laser medium; and a first laser diode and a second laser diode arranged for projecting pumping lights to the first laser medium and the second laser medium respectively. The first laser medium is arranged such that the progress direction of a laser beam generated by the first and the second laser medium is substantially in parallel with the Ng axis of the first laser medium and the polarization direction of a laser beam generated by the first and the second laser medium is substantially in parallel with the Np axis of the first laser medium. The second laser medium is arranged such that the progress direction of the laser beam generated by the first and the second laser medium is substantially in parallel with the Np axis of the second laser medium and the polarization direction of the laser beam generated by the first and the second laser medium is substantially in parallel with the Nm axis of the second laser medium.

Description

Femtosecond laser apparatus and femtosecond laser system including the femtosecond laser apparatus and a femtosecond laser system including the femtosecond laser apparatus,

The present application relates to a laser device, and more particularly, to a femtosecond laser device and a femtosecond laser system including the same that can increase the laser output power and at the same time improve the quality of the pulse beam.

Ultrashort laser sources, such as femtosecond (fs) laser pulses, generate ultrashort pulses with high peak powers and have high pulsed average powers, resulting in ultra-fast spectroscopy, high energy physics, XUV-wave It is not only widely used in the basic science field, but also its applicability can be confirmed in various fields such as ultrafine laser processing and microsurgery.

In general, the ultrasound laser pulses have excellent characteristics such as high peak power as well as short pulse time width, wide spectrum bandwidth and the like.

Since such ultra-short laser pulses can be applied to micro or nano processing of electronic components and optical components requiring high precision such as solar cells, optical memories, semiconductors, and flat panel displays, the demand for industrial ultra-short pulse laser systems is increasing have.

In order to meet the above requirements, first, conditions for application of ultraraple laser pulses to ultrafine laser processing will be described.

First, the laser pulse time width is much shorter than the electron-phonon relaxation time of the target material so that the thermal energy should not be transferred to the vicinity of the part to be processed (non-thermal processing).

This is called cold ablation.

For example, the electron-phonon relaxation time of aluminum is 4.27 picosecond (ps), iron 3.5 ps, and copper 57.5 ps.

That is, in the case of ultra-fine laser machining of aluminum, it is preferable to apply a laser pulse with a pulse time width of less than a picosecond for cooling ablation.

Therefore, femtosecond lasers are the best lasers for ultrafine laser processing of cooling ablation.

Ultrathinary laser pulses in the femtosecond range can process extremely hard materials that are difficult to mechanically process because they minimize thermal diffusions in the machining area and do not damage the surroundings due to residual heat. In addition, they can process very hard materials with short pulse width and high pulse energy. it is possible to process ultra-precise structures with various nanometer scales, even transparent materials such as glass and polymers, due to the nonlinear optical effect of multi-photon absorption due to its high peak power.

Second, the target materials to be processed have an ablation threshold value of about several J / cm2 or more. Considering the size of the laser beam focused on the processing region for ablation processing, a pulse energy of about 10 μJ or more is required.

Some cases of material processing applications require pulse energies of several hundreds of μJ.

As a typical laser having such excellent characteristics, there is a titanium sapphire laser (Ti: sapphire laser).

Commercially available titanium sapphire lasers to date provide a pulse time width of several to several hundreds of femtoseconds, pulse energies of several mJ or a few J.

However, it is difficult to obtain a pulse repetition rate higher than several tens of kHz because an expensive high-power pulse green laser such as Nd: YVO ₄ laser should be used as a pumping light source.

In addition, titanium sapphire lasers are problematic in that they are large in size, expensive, and difficult to maintain a stable pulse output, making them difficult to use in a production site.

In a diode-pumped solid-state (DPSS) laser, a pumped light source such as a laser diode is used as a pumping light source, and a femtosecond laser is constructed using a solid-state laser medium. The size of the laser head becomes small and the laser diode of a wavelength widely used in various commercial fields is low in cost compared to the output, and the cost of the femtosecond laser can be lowered, thereby reducing the cost.

In addition, the solid laser has a short optical pumping distance and is capable of stable laser operation, which is very advantageous for application to industrial lasers.

In recent years, the development of solid state laser systems using diode pumping has been rapidly growing as laser diode arrays and laser diode bars capable of achieving high output with high efficiency and high power have been developed due to improvements in semiconductor and electronic engineering techniques.

In order to realize a femtosecond laser system that pumps light with such a laser diode, it is essential to design and manufacture a laser pumping module suitable for the optical pumping module.

The laser medium for diode pumping mainly uses crystals doped with rare earth ions such as neodymium (Nd) and ytterbium (Yb), which can be pumped by laser diodes of 808 nm and 980 nm, respectively.

In the early development stage of high power lasers, neodymium-doped laser crystals have been preferred because they have a four-level structure and various absorption lines. In recent years, ytterbium-doped crystals having simpler energy levels have thermally and optically superior properties It is being used in many ways.

 There are additional requirements to apply a femtosecond laser source to industrial applications such as ultrafine laser machining.

For example, if the pulse repetition rate of the laser is low, it takes a lot of time to process the laser, which lowers productivity on the production site.

However, although the pulse repetition rate of the laser is preferably high, there are also limitations in increasing the pulse repetition rate.

If the pulse repetition rate is too high so that the plasma generated by the femtosecond laser pulse is turned off before the next laser pulse is turned on, the next laser pulse is generated by the plasma present at the target site, And the like.

This is called plasma shielding.

In order to suppress the plasma shielding effect, the next laser pulse must be applied after the plasma relaxation time has elapsed.

That is, the time interval between the laser pulse and the next laser pulse must be longer than the plasma relaxation time. The plasma relaxation time varies depending on the medium to be processed, but the repetition rate of the laser pulse repetition rate is about 1 MHz.

Therefore, a femtosecond laser with a pulse repetition rate of several hundreds of kHz is required to maintain high productivity in a production site.

Further, in order to mount and operate a laser light source in a laser processing system, it is required to have a compact size, a low price, and a high operation stability that does not change the laser operation state for a long time.

When the femtosecond pulse is first generated by mode locking in a femtosecond oscillator, the pulse energy is very low as nano joules (nJ), which is not suitable for applications such as laser processing.

To increase the femtosecond pulse energy, chirped pulse amplification (CPA) technology is used.

For example, a pulse stretcher is used to extend a pulse from a femtosecond oscillator in a time-wise manner and then applied to an amplifier to amplify the pulse energy.

Then, the amplified pulse is passed through a pulse compressor to return the time width of the pulse to the original femtosecond region.

At this time, pulses from the femtosecond oscillator serve as seeding pulses applied to the amplifier.

In the pulse expander, a chirping phenomenon in which a pulse is elongated in time due to a path difference due to a wavelength is referred to as a chirp pulse amplification technique.

By using this technique, the peak output of the pulse amplified in the resonator of the pulse amplifier can be kept low to suppress the nonlinear distortion occurring in the temporal or spatial distribution of the laser pulse by the self-focusing effect, It also prevents physical damage to the optical components that make up the system.

That is, it is possible not only to prevent damage to the system due to the high energy laser pulse but also to operate the pulse amplifier efficiently to increase the pulse energy.

In recent years, in a MOPA system that combines a femtosecond master oscillator (MO) that directly pumps a diode light source based on chirp pulse amplification technology and a power amplifier (PA) that pumps a diode light source directly, As energy becomes available, significant progress has been made in the development of femtosecond laser systems with high peak power and high average power.

However, the laser medium doped with ytterbium has a two-level energy structure or a quasi-three-level energy structure, so that light emitted at an optical pumping wavelength of 981 nm is again absorbed by the laser medium.

To overcome this, we concentrate the light generated by the high-power laser diode with a high output in a very small spot size in the laser crystal.

In this process, the pumping light which can not be deformed by the laser beam is transmitted around the spot of the laser crystal in the form of heat energy and also transferred to the mount of the laser crystal.

If a large amount of thermal energy is accumulated in this process, the amplified laser beam is distorted, resulting in deteriorated beam quality, and the laser average power and pulse energy are also limited.

If the thermal energy accumulated in the laser crystal is higher than the damage threshold, there is a problem that laser oscillation is stopped due to physical damage such as cracking of the laser crystal or breaking of the laser crystal.

Meanwhile, previous studies for generating or amplifying femtosecond pulses using a plurality of Yb: KYW or Yb: KGW laser crystals will be described.

For example, U.S. Patent No. 7,508,847 B2 proposes a concept of increasing the number of times a laser beam passes through a gain medium in a resonator by optically pumping two anisotropic media such as Yb: KGW.

However, this patent focuses on extending the length of the laser resonator to reduce the instability of the pulse power called "Triggered mode", and proposes two gain medium pumping schemes as one of the various forms of this long resonator But does not present experimental implementations or results.

In addition, US Patent 6,760,356 B2 proposes a concept of amplifying a femtosecond pulse using two Yb: YAG isotropic laser crystals.

However, these two prior patents are described only for amplifying the output using only two laser media, and for the thermal, optical and other properties to be considered along the optical axis of the laser medium in other femtosecond laser systems . That is, in the two patents, there is no mention of the axis of the laser medium, but only the effect of increasing the output power by increasing the number of laser media is described.

On the other hand, a technique for efficiently picking up depolarized pumping light by selecting the axis of the Yb: KYW laser medium and adjusting the polarization ratio of the pumping light pumping the laser medium is proposed in U.S. Patent No. 6,891,876 B2.

This patent concentrates mainly on optical pumping of anisotropic media. Firstly, even when using the depolarized pumping light, the axis of the anisotropic gain medium and the wavelength of the pumping laser are selected to optically pump the laser medium efficiently I have suggested how to do it.

Secondly, even when the wavelength of the pumping light is unstable, a method of appropriately selecting the axis of the laser medium and controlling the polarization ratio of the pumping laser incident on the laser medium does not cause a significant change in the laser output.

The proposed method is based on the fact that the absorption spectrum along the axial direction of the laser medium is different, so that the direction of the laser medium is determined so that the two axes are properly mixed in one laser medium, and the intensity of the pumping light is adjusted according to the polarization direction It has been shown that it is possible to have a similar absorption cross-section in a wide wavelength region.

However, in such a case, it is advantageous to fabricate an optical pumping section which is less sensitive to the degree of polarization or wavelength instability of the pumping light, but in order to have a similar absorption cross section in a wide wavelength region, optical pumping should be performed in a wavelength region where the absorption cross section of the laser medium is small .

In addition, since the output is dispersed to two polarized beams by one laser light source, the pumping efficiency is not only reduced but also the laser wavelength is not converted. The absorbed pumping light is accumulated in the laser medium as thermal energy, There is a disadvantage that it is also limited.

In addition, since the Yb: KYW or Yb: KGW laser crystal has an advantage in producing a femtosecond laser having a high average output power because of its excellent thermal conductivity, the anisotropic laser medium of Yb: KYW or Yb: When the average power of the laser is increased due to the different thermal conductivity, astigmatism of the thermal lens due to the thermal effect occurs, which causes the laser beam shape to be distorted and the beam quality to deteriorate.

Also, during the process of generating or amplifying the femtosecond pulse using the plurality of laser crystals, the pumping light generated from the laser diode is incident on and absorbed by one laser crystal, and the remaining pumping light, which has not been absorbed, So that the alignment characteristics of the pumping light and the laser beam are degraded.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a laser processing method and a laser processing method which use a plurality of laser media and make the laser beam generated from the laser medium substantially parallel to a specific axis of the laser medium, And a femtosecond laser system including the femtosecond laser system.

Also, a laser device and a femtosecond laser system including the laser device capable of shortening a time width of a pulse ultimately generated by a laser system by applying spectral-shaped pulses from the outside of the resonator as a seed pulse to suppress gain narrowing in the amplification process are provided It has its purpose.

The present invention also provides a laser apparatus and a femtosecond laser system including the same, which can broaden a gain spectrum bandwidth by making the polarization direction of a laser beam generated from a laser medium substantially parallel to a specific axis of each of a plurality of laser mediums, There is a purpose.

It is another object of the present invention to provide a laser device and a femtosecond laser system including the laser device capable of broadening the spectrum width of pulses by superimposing a gain spectrum of a plurality of laser media having different wavelengths corresponding to the maximum value of the radiation sectional area.

The present invention also provides a laser device capable of preventing degradation of alignment characteristics of pumping light and a laser beam generated during a process of generating or amplifying a femtosecond pulse using the plurality of laser media in a femtosecond pulse generation process using a plurality of laser media, And to provide a femtosecond laser system including the same.

According to an aspect of the present invention, there is provided a femtosecond laser device including: a first laser medium having spatially mutually perpendicular Ng, Np and Nm axes; An Np axis substantially parallel to the Ng axis of the first laser medium, an Nm axis substantially parallel to the Np axis of the first laser medium, and an Ng axis substantially parallel to the Nm axis of the first laser medium A second laser medium; And a first laser diode and a second laser diode arranged to respectively irradiate the first laser medium and the second laser medium with pumping light, wherein the first laser medium comprises a first laser medium and a second laser medium, Wherein the polarizing direction of the laser beam generated from the first and second laser media is disposed on the Np axis of the first laser medium so that the polarization direction of the generated laser beam is substantially parallel to the Ng axis of the first laser medium, And the second laser medium is disposed such that a traveling direction of a laser beam generated from the first and second laser media is substantially parallel to an Np axis of the second laser medium, And the polarization direction of the laser beam generated from the second laser medium is substantially parallel to the Nm axis of the second laser medium.

Another femtosecond laser device according to an embodiment of the present invention includes a first laser medium having spatially mutually perpendicular Ng, Np and Nm axes; An Np axis substantially parallel to the Ng axis of the first laser medium, an Ng axis substantially parallel to the Np axis of the first laser medium, and an Nm axis substantially parallel to the Nm axis of the first laser medium A second laser medium; And a first laser diode and a second laser diode arranged to respectively irradiate the first laser medium and the second laser medium with pumping light, wherein the first laser medium comprises a first laser medium and a second laser medium, Wherein the direction of the generated laser beam is substantially parallel to the Ng axis of the first laser medium, and the direction of polarization of the laser beam generated from the first and second laser media is parallel to the Nm axis of the first laser medium And the second laser medium is disposed such that a traveling direction of a laser beam generated from the first and second laser media is substantially parallel to an Np axis of the second laser medium, And the polarization direction of the laser beam generated from the second laser medium is substantially parallel to the Nm axis of the second laser medium.

Another femtosecond laser apparatus according to an exemplary embodiment of the present invention includes a first laser medium and a second laser medium having spatially mutually perpendicular Ng, Np, and Nm axes and disposed to face each other; A first laser diode and a second laser diode arranged to respectively inject pumping light into the first laser medium and the second laser medium; Wherein the polarization direction of the laser beam generated from the first and second laser media is different from that between the Np axis of the first laser medium and the Nm axis of the second laser medium Wherein the first laser medium is disposed such that a traveling direction of a laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium, The second laser medium is disposed so that the traveling direction of the laser beam generated from the first and second laser media is substantially parallel to the Np axis of the second laser medium.

Another femtosecond laser apparatus according to an exemplary embodiment of the present invention includes a first laser medium and a second laser medium having spatially mutually perpendicular Ng, Np, and Nm axes and disposed to face each other; A first laser diode and a second laser diode arranged to respectively inject pumping light into the first laser medium and the second laser medium; Wherein a polarization direction of the laser beam generated from the first and second laser media is different from a polarization direction of the laser beam generated from the Nm axis of the first laser medium and the Nm axis of the second laser medium Wherein the first laser medium is disposed such that a traveling direction of a laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium, The second laser medium is disposed so that the traveling direction of the laser beam generated from the first and second laser media is substantially parallel to the Np axis of the second laser medium.

The femtosecond laser device according to an embodiment of the present invention can improve the laser output intensity as well as the quality of the laser beam generated in the laser medium.

In addition, the femtosecond laser device according to an embodiment of the present invention can reduce the pulse time width by modifying the pulse spectrum into a desired shape.

In addition, the femtosecond laser device according to an embodiment of the present invention can change the spectral width by superimposing a gain spectrum of a plurality of laser media having different wavelengths corresponding to the maximum value of the radiation sectional area.

Also, the femtosecond laser device according to an embodiment of the present invention can improve the alignment characteristics of the pumping light and the laser beam.

1 is a schematic diagram illustrating a femtosecond laser system according to an embodiment of the present invention.
FIG. 2 is a graph for explaining spectral characteristics according to a gain narrowing of an amplified and outputted pulse according to an embodiment of the present invention. Referring to FIG.
FIG. 3 is a graph for explaining a spectral change of an output pulse according to spectral shaping using a spectral shaping machine according to an embodiment of the present invention. Referring to FIG.
Fig. 4 is a diagram showing an example in which when the polarization direction of the laser beam generated from the laser crystal Yb: KYW is parallel to the Nm-axis or Np-axis direction of the laser crystal Yb: KYW, Fig. 8 is a graph showing the spectral characteristics of the radiation cross-sectional area of a crystal.
FIG. 5 is a diagram illustrating spectral characteristics obtained by combining spectra of the radiation cross-sectional area of FIG. 4 according to an embodiment of the present invention.
6 is a schematic diagram of a laser beam generator according to an embodiment of the present invention.
7 is a schematic view of another laser beam generator according to an embodiment of the present invention.
8 is an optical conceptual diagram of a femtosecond laser apparatus for explaining an arrangement structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to an embodiment of the present invention.
9 is a diagram illustrating laser beam characteristics experimentally performed according to an arrangement structure of a laser medium according to an embodiment of the present invention.
10 is an optical conceptual diagram of a femtosecond laser apparatus for explaining an arrangement structure of a laser medium for improving an average output of a laser beam according to another embodiment of the present invention.
11 is an optical conceptual diagram of another femtosecond laser device for explaining a layout structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to an embodiment of the present invention.
12 is an optical conceptual diagram of another femtosecond laser apparatus for explaining an arrangement structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to another embodiment of the present invention.
13 is a view for explaining a laser beam generator included in a master oscillator or an amplifier according to an embodiment of the present invention.
14 is a view for explaining a beam damper structure according to an embodiment of the present invention.
15 is a graph illustrating the slope efficiency of a continuous wave output according to the intensity of pumping light applied to a laser medium according to an embodiment of the present invention.
16 is an optical conceptual diagram showing a spectral shaping machine according to an embodiment of the present invention.
17 is a graph showing spectra before and after spectral shaping of a seed pulse according to an embodiment of the present invention.
18 is a graph showing a spectrum of pulses amplified without spectral shaping and spectrally shaped and amplified pulses according to an embodiment of the present invention.
Fig. 19 is a graph showing pulse time widths of a pulse amplified without spectral shaping and a spectrum-amplified pulse, according to an embodiment of the present invention.
FIG. 20 is a graph showing a change in pulse energy according to the pulse repetition rate in the embodiment of the present invention. FIG.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, which will be easily understood by those skilled in the art.

1 illustrates a femtosecond laser system in accordance with an embodiment of the present invention.

1, a femtosecond laser system 100 includes a master oscillator 110, a paradier isolator 120, a pulse expander / compressor 130, a spectral shaping unit 140, a thin film polarizer (TFP) 150). An amplifier 170, and a pulse picker 160.

The master oscillator 110, although not shown in the figure, includes a laser device capable of generating a laser beam therein, such as the amplifier 170, to generate an ultrasensitive pulse in the femtosecond region.

The master oscillator 110 may use a fiber laser or a solid state laser for laser beam generation. The laser medium used in solid state lasers can be selected from a variety of laser media depending on their thermal, optical and mechanical properties.

For example, the laser medium may be an amorphous medium or a crystalline medium, and when using a crystalline medium, at least one of an isotropic crystal, an uniaxial crystal and an anisotropic crystal of a biaxial crystal You can use one.

At least one of Yb: YAG, Yb: ScO, Yb: YO, Yb: LuO, Yb: LuScO, and Yb: CaF may be used as the amorphous medium, decision is _{Yb: CALGO, Yb: YVO 4} , Yb: NGW, Yb: NYW, Yb: LuVO, Yb: LSB, Yb: S-FAP, Yb: can be used at least one of a C-FAP, twin crystals Yb: KYW, Yb: KGW, Yb: KLuW, Yb: YCOB, Yb: YAP

The pulse expander / compressor 130 includes an expander 131 and a compressor 132 to expand a femtosecond pulse width generated by the master oscillator 110 or to compress a pulse amplified by the amplifier into a pulse of a femtosecond region .

The expander 131 and the compressor 132 may respectively expand or compress the pulse using a separate spectroscopic element such as a grating.

In addition, the expander 131 and the compressor 132 may be integrally formed so as to expand and compress the pulse using one spectroscopic element in common. Accordingly, the size of the femtosecond laser system 100 can be made compact, and the manufacturing cost can be reduced.

The expander 131 extends the width of the femtosecond pulse generated in the master oscillator 110 in a time-wise manner to prevent physical damage to optical components such as a laser medium during the amplification of the femtosecond pulse by the amplifier 170 .

For example, the expander 131 expands a pulse of about 100 femtoseconds (fs) generated by the master oscillator 110 to several tens of picoseconds (ps).

The compressor 132 compresses the pulse amplified by the amplifier 170 into pulses of a femtosecond range and delivers the compressed pulse to the outside.

A Faraday isolator 120 is disposed between the master oscillator 110 and the pulse expander / compressor 130 to prevent pulses of high energy generated in the amplifier 170 from being incident on the master oscillator 110 .

A spectral shaper 140 transforms the spectrum of the pulse into a desired shape for an extended pulse in a pulse expander 131. [ That is, the spectrum shaping unit 140 can form a spectrum of seeding pulses input to the amplifier 170 to compensate for the narrowing of the spectral band width in the process of amplification in the amplifier. Here, the seed pulse means a pulse applied to the amplifier 170 for pulse amplification.

This spectrum shaping device 140 may be included in the master oscillator 110 or the amplifier 170 or may be incorporated into the femtosecond laser system 110 when the spectrum of the pulse is transformed into a desired shape in the master oscillator 110 or the amplifier 170. [ (100).

The spectral shaping of the pulses will be described later.

The spectrally shaped pulses are applied to the amplifier 170 via a total reflection mirror (FM), a thin film polarizer (TFP) and a Faraday rotator (150). At this time, the total reflection mirror FM is for changing the beam path, and can be inserted or deleted according to the size and design conditions of the femtosecond laser system. However, when the total reflection mirror FM is used as in the embodiment of the present invention, the path of the beam can be changed even in a limited space, so that the femtosecond laser system can be made compact.

The amplifier 170, like the master oscillator 110, includes a laser beam generator using a fiber laser or a solid laser, and amplifies the energy of the input seed pulse using the laser beam generator.

The laser medium used in the amplifier 170 may use the same laser medium used in the master oscillator 110 or may use a different laser medium.

That is, the laser medium used in the master oscillator 110 and the amplifier 170 can be made in various combinations between the crystalline medium and the amorphous medium, and both of the laser medium used in the master oscillator 110 and the amplifier 170 can be used as the crystal medium , It can be formed by various combinations of isotropic crystals and anisotropic crystals.

Since the specific laser medium that can be used in the amplifier 170 is substantially the same as the laser medium that can be used in the master oscillator 110, a description thereof will be omitted.

The paradigal rotation unit 150 rotates the polarization direction of the laser pulse 90 degrees so that the traveling path of the laser pulse amplified by the amplifier 170 is changed toward the pulse picker 160 by the thin film polarizer TFP .

The pulse picker 160 includes an electro-optic switch and switches the switch to selectively pass necessary pulses and unnecessary pulses for the amplified laser pulses. Thereafter, the selected laser pulse is compressed by the compressor 132 into the ultrasound laser pulse in the femtosecond region and is emitted outside the femtosecond laser system.

As described above, the femtosecond laser system 100 according to the present invention amplifies the energy of the laser pulses by amplifying the energy of the laser pulses so that the laser pulses of several nJ regions generated in the master oscillator 110 can be easily applied to femtosecond laser processing.

On the other hand, since the laser medium has a gain profile having a fundamentally limited width, the magnification to be amplified varies depending on the wavelength of the incident pulse, and the gain narrowing in which the spectrum bandwidth of the amplified pulse is narrowed gain narrowing occurs. As a result, there arises a problem that the time width of the pulse is widened.

FIG. 2 is a graph for explaining spectral characteristics according to a gain narrowing of an amplified and outputted pulse according to an embodiment of the present invention. Referring to FIG.

Referring to FIG. 2, when an input pulse having a spectrum as shown in (a) is applied to a laser medium having a gain profile of a limited width as shown in (b) In the central wavelength (λc), the amplification is continued, but the gain at the off-center edge wavelength is low, so that the amplification ratio becomes smaller at the edge wavelength than at the center wavelength.

 That is, if the input pulse travels back and forth in the resonator of the amplifier, the intensity of the amplified signal increases as the number of passes through the laser gain medium increases, and the intensity at the edge wavelength becomes relatively lower than the center wavelength.

In the graph of FIG. 2, it can be seen that the width of the output pulse spectrum of (c) becomes narrower than the width of the input pulse spectrum of (a).

Thus, if the spectral band width of the laser pulse is narrowed, the time width of the laser pulse is widened so that the heat is diffused in the interaction between the laser beam output from the femtosecond laser system and the workpiece, do.

Accordingly, in one embodiment of the present invention, the spectral shaping unit 140 may be used to form a spectrum having a desired shape before applying the seed pulse in the amplifier 170 to reduce the time width of the laser pulse, or the master oscillator 110 ) Or the optical axis of the plurality of laser media in the amplifier 170 to prevent narrowing of the spectral band width.

Methods for broadening the spectral bandwidth of a pulse include spatial dispersive amplification, a method of changing the spectrum by inserting an optical element in the resonator, and nonlinear pulse compression.

FIG. 3 is a graph for explaining a spectral change of an output pulse according to spectral shaping using a spectral shaping machine according to an embodiment of the present invention. Referring to FIG.

Referring to FIG. 3, before applying the input pulse having the spectrum shown in (a) to the laser medium having a gain profile of a limited width as in (b), the intensity at the center wavelength And spectrum shaping (SS) is performed in a high-intensity M-shape at a wavelength slightly deviated from the center.

When a pulse with such a modified spectrum is applied to a laser medium having a gain profile as in (b), the gain is low at the center wavelength and high at the wavelength slightly off center.

When the laser pulse oscillates back and forth in the resonator of the amplifier 170, the number of times of passing through the laser gain medium increases, and the difference in the ratio amplified in accordance with the wavelength accumulates, resulting in an output pulse having the same output pulse spectrum as in (d) .

When the output pulse spectrum d is normalized, it can be seen that its band width is wider than the band width of the input pulse spectrum of (a) as shown in (e).

In order to prevent the spectral band width from being reduced, a plurality of laser mediums are used for the laser oscillator included in the master oscillator 110 or the amplifier 170 as well as the spectral distortion of the pulse using the spectrum shaping machine, And can spectrally combine the inherent gain spectra of each of the plurality of laser media.

That is, the gain spectrum of a plurality of laser mediums having different wavelengths corresponding to the maximum value of the radiation sectional area can be overlapped to broaden the spectral bandwidth of the pulse.

At this time, the laser medium to be used can be an amorphous medium, a crystal medium, an amorphous medium and an amorphous medium, a crystal medium and a crystal medium, and the number of laser mediums is not limited. In addition, the crystalline medium may include both an isotropic medium and an anisotropic medium. 4 and 5, the laser generating apparatus uses a plurality of laser media of the same kind and uses characteristics different in the spectrum of the radiation cross-sectional area along the optical axis, so as to broaden the spectral bandwidth of the pulse, The spectrum of the laser medium along the optical axis can be overlapped to broaden the spectral bandwidth of the pulse.

For example, when the laser medium is anisotropic laser crystal such as Yb: KYW or Yb: KGW, the direction of polarization of the laser beam generated from the anisotropic laser crystal is parallel to an axis direction of the crystal, -section) have different spectra.

Experimentally, when the polarization direction of the laser beam in the wavelength range of 1015 to 1050 nm is substantially parallel to the Nm axis of the laser crystal, based on optical axes such as Nm, Np and Ng of the Yb: KYW laser crystal Axis and is substantially larger when it is substantially parallel to the Np-axis, and is substantially 10 times smaller than when it is substantially parallel to the Nm-axis when it is substantially parallel to the Ng-axis.

These experiments can extend the spectral bandwidth of the pulses using crystallographic axes such as the a-, b-, and c-axes, rather than the optical axis.

FIG. 4 is a graph showing the relationship between the polarization direction of the laser beam generated from the laser crystal (Yb: KYW) and the Nm-axis or Np-axis direction of the laser crystal (Yb: KYW) FIG. 5 is a graph showing spectral characteristics obtained by combining the spectrum of the radiation cross-sectional area of FIG. 4; FIG.

4, when the polarization direction of the laser beam generated from the laser crystal (Yb: KYW) is substantially parallel to the Nm-axis of the laser crystal (Yb: KYW), when the wavelength is near 1025 nm, (A), and when the polarization direction of the laser beam generated from the laser crystal (Yb: KYW) is substantially parallel to the Np-axis of the laser crystal (Yb: KYW) The radiation cross-section has a maximum value at around 1040 nm, and shows the spectrum distribution as shown in (b).

Referring to FIG. 5, (a) shows the spectrum of the radiation cross-sectional area of FIG. 4 (a) and (b) combined at a ratio of 1: 1 to broaden the spectrum width, To broaden the width, the spectrum of the radiation cross-sectional area of FIGS. 4 (a) and 4 (b) is combined at 1: 3.

5 shows an example in which the spectral bandwidth of the radiation sectional area can be widened. However, it is possible to broaden the spectrum bandwidth by variously combining the spectral characteristics of different radiation sectional areas. At this time, the spectral coupling preferably uses an optical axis having a large radiation cross-sectional area so as to increase the output of the laser beam generated from the laser crystal.

Meanwhile, since the Yb: KYW or Yb: KGW laser crystal has not only the radiation cross-sectional area but also the absorption cross-sectional area depending on the specific axial direction, the polarization direction of the pump light can be combined in various forms to improve the optical pumping efficiency.

Preferably, the absorption cross-sectional area in the Ng-axis direction is about 10 times smaller than the absorption cross-sectional area in the Nm-axis direction and the absorption cross-sectional area in the Nm-axis direction is about 5 times larger than the absorption cross- Axis is substantially parallel to the Nm-axis of the laser crystal.

6 is a schematic diagram of a laser beam generator according to an embodiment of the present invention.

As shown, the laser beam generator 200 can be included in a master oscillator (not shown) and includes laser media C1 and C2, a dichroic mirror DM, a focusing lens FL, a collimating lens CL A first wavelength plate 24, a laser diode 21, an optical fiber 22, and a beam damper 30.

Laser diodes 21 are disposed on both sides of the laser mediums C1 and C2 and the laser diodes 21 are optically connected to the optical fibers 22, respectively. The laser diode 21 is a light source for generating pumping light and applies pumping light to the laser mediums C 1 and C 2 through the respective optical fibers 22.

The types of the laser media C1 and C2 and the arrangement of the laser media C1 and C2 can be variously combined in consideration of spectral characteristics, thermal characteristics, efficiency, and output of pulses.

The first wave plate 24 is a half wave plate (1/2) and is disposed after each optical fiber 22 along the path of the pumping light to adjust the polarization direction of the light generated in the laser diode 21.

The collimating lens CL and the focusing lens FL are sequentially disposed after the first wave plate 24 along the path of the pumping light so that the pumping light polarized at the first half wave plate 24 is reflected by the laser medium (C1, C2).

The dichroic mirror DM reflects the laser beam generated through the laser mediums C1 and C2 and transmits the laser light through the laser mediums C1 and C2 in order to transmit the pumping light generated from the laser diode 21. [ And is disposed on the front and rear sides.

On the other hand, the first wavelength plate 24, the collimating lens CL, the focusing lens FL, the dichroic mirror DM, and the laser mediums C1 and C2 are not shown in the drawing, For example, even when there is a change in temperature or humidity, the pumping light generated from the laser diode can be fabricated in the form of an integrated optical pumping module through mechanical component fastening so as to maintain the beam alignment characteristic of entering the laser medium

The beam damper 30 is disposed between the laser media C1 and C2 to prevent degradation of the alignment characteristics of the pumping light and the laser beam generated in the process of amplifying the laser pulse. A detailed description of the beam damper will be described later.

In addition, optical components capable of adjusting the beam path, such as concave mirrors, convex mirrors, total reflection mirrors, etc., can be added to the laser generator to adjust the path of the laser beam.

7 is a schematic view of another laser beam generator according to an embodiment of the present invention.

As shown, the laser beam generator 300 can be included in an amplifier (not shown) and includes laser media C1 and C2, a dichroic mirror DM, a focusing lens FL, a collimating lens CL, The first wavelength plate 24, the second wavelength plate 25, the thin film polarizer TFP, the concave mirrors CM1 and CM2, the total reflection mirror FM, the laser diode 21, the optical fiber 22, (23) and a beam damper (30).

The laser beam generator is almost similar to the laser beam generator of FIG. Therefore, description of similar optical components will be omitted.

However, the concave mirrors CM1 and CM2 and the total reflection mirror FM serve to change the path of the laser beam. The number and position of the concave mirrors CM1 and CM2 and the total reflection mirrors FM may vary depending on the scale of the laser beam generator or the moving distance of the beam path. In the embodiment of the present invention, only the concave mirror and the total reflection mirror are described as the beam path conversion means, but any other beam path conversion means such as a convex mirror, a flat plate mirror,

The Pockels cell 23, the second wave plate 25 and the thin film polarizer TFP are disposed in the optical path direction and serve as a switch for drawing out the laser beam generated in the laser medium to the outside. Here, the second wavelength plate 25 is a 1/4 wavelength plate. For example, a voltage of several kV is applied to the Pockels cell 23 to change the polarization direction of the laser beam amplified through the optical components.

8 is an optical conceptual diagram of a femtosecond laser apparatus for explaining an arrangement structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to an embodiment of the present invention.

As shown, the first and second laser media C1 and C2 each have a crystal plane based on an optical axis, and each crystal plane is formed in a direction perpendicular to the optical axis. That is, the first and second laser media C1 and C2 have crystal planes having spatially mutually perpendicular Ng, Np and Nm axes, respectively, and the first laser medium C1 and the second laser medium C2 They are arranged differently.

The second laser medium C2 includes an Np axis substantially parallel to the Ng axis of the first laser medium C1, an Nm axis substantially parallel to the Np axis of the first laser medium C1, Axis and an Ng axis substantially parallel to the Nm-axis of the laser beam generator C1.

Although not shown in the drawing, the first and second laser mediums C1 and C2 are disposed adjacent to the first laser medium C1 and the second laser medium C2, respectively, so that pumping light is incident on the first and second laser media C1 and C2. 2 laser diodes are arranged.

The first laser medium C1 and the second laser medium C2 are arranged so that the traveling direction of the laser beam generated in the first and second laser media C1 and C2 is substantially parallel to the Ng axis of the first laser medium And is disposed substantially parallel to the Np-axis of the second laser medium C2.

The first laser medium C1 and the second laser medium C2 are arranged so that the first and second laser mediums C1 and C2 are in resonance in the laser beam generator so that the spectral bandwidth can be widened, (C2) of the second laser medium (C2) so as to be substantially parallel to the Np axis of the first laser medium (C1) and substantially parallel to the Nm axis of the second laser medium (C2) . Accordingly, the laser beam advances in the axial direction of the laser medium having different thermal characteristics, and the quality of the beam can be improved without distorting the beam according to the thermal effect.

In order to increase the efficiency, the first laser medium C1 and the second laser medium C2 are arranged such that the polarization direction Epump of the first pumping light generated from the first laser diode is Nm Axis so that the polarization direction Epump of the second pumping light generated from the second laser diode is substantially parallel to the Nm axis of the second laser medium C2.

That is, the polarization direction of the laser beam generated from the first laser medium C1 and the second laser medium C2 is determined by taking into account the radiation cross-sectional area, the Nm-axis of any one of the first and second laser media C1 and C2 Axis and is substantially parallel to the remaining one Np-axis, thereby widening the spectral bandwidth and at the same time preventing beam distortion due to the thermal effect. In addition, the polarization direction (Epump) of the pumping light generated from the first and second laser diodes is substantially parallel to the Nm-axis of the first laser medium (C1) and the second laser medium (C2) Thereby improving the efficiency.

In one embodiment of the present invention, the thermal properties of the laser medium are primarily compensated for by using two laser media to improve beam quality and to combine different gain spectral distributions to broaden the band width, Three or more laser media can be used to widen the pulse width to reduce the pulse width. The arrangement of the laser media and the laser system including it can be freely embodied in any form.

9 is a diagram illustrating laser beam characteristics experimentally performed according to an arrangement structure of a laser medium according to an embodiment of the present invention.

9 (a), 9 (b) and 9 (c), the laser medium size is equal to 2 × 2 × 5 mm ³ , the doping ratio of Yb is 3% 15W and the polarization direction Epump of the pumping light are substantially parallel to the Nm axis of the first laser medium and the second laser medium, respectively.

9A is a graph showing the relationship between the polarization direction of the laser beam and the polarization direction of the laser beam in the case of the first laser medium in such a manner that the traveling direction of the laser beam is substantially parallel to the Ng axis of the first laser medium, Elaser) is substantially parallel to the Np-axis of the first laser medium, and in the case of the second laser medium, the traveling direction of the laser beam is substantially parallel to the Ng-axis of the second laser medium And the polarization direction (Elaser) of the laser beam is arranged substantially parallel to the Nm-axis of the second laser medium. As can be seen from the figure, the shape of the laser beam is elliptical, and the beam is distorted, thereby deteriorating the beam quality.

FIG. 9B shows a case where the direction of advance of the laser beam is substantially parallel to the Ng axis of the first laser medium and the polarization direction (Elaser) of the laser beam is Nm Axis of the second laser medium is substantially parallel to the Np axis of the second laser medium in the case of the second laser medium and the polarization direction (Elaser) of the laser beam is Nm Lt; RTI ID = 0.0 > parallel < / RTI > As can be seen from the figure, the shape of the laser beam is elliptical, and the beam is distorted, thereby deteriorating the beam quality.

On the other hand, in the case of the first laser medium, as shown in (c) of FIG. 9, the laser beam advancing direction is substantially parallel to the Ng axis of the first laser medium, and the polarization direction (Elaser) The direction of the laser beam is substantially parallel to the Np axis of the second laser medium and the polarization direction of the laser beam is in a direction substantially parallel to the Np axis of the second laser medium, Is a beam characteristic that appears when disposed substantially parallel to the Nm axis of the medium. As shown in the figure, the shape of the beam is close to a circle without distortion, and the beam quality is improved.

On the other hand, it is preferable to use a femtosecond laser with a high average power in consideration of the material to be processed or the environment of the production site and the stability of the laser system. Therefore, in one embodiment of the present invention, as shown in FIG. 10, the arrangement structure of the laser medium may be different for improving the average power of the beam.

10 is an optical conceptual diagram of a femtosecond laser apparatus for explaining an arrangement structure of a laser medium for improving an average output of a laser beam according to another embodiment of the present invention.

10 is similar to the optical conceptual diagram of the femtosecond laser device of FIG. That is, the second laser medium C2 includes an Np axis substantially parallel to the Ng axis of the first laser medium C1, an Ng axis substantially parallel to the Np axis of the first laser medium C1, Axis and an Nm axis substantially parallel to the Nm-axis of the laser beam source C1.

The first laser medium C1 and the second laser medium C2 are arranged such that the traveling direction of the laser beam generated in the first and second laser media C1 and C2 is substantially parallel to the Ng axis of the first laser medium C1 And arranged so as to be substantially parallel to the Np-axis of the second laser medium C2. The first laser medium C1 and the second laser medium C2 are arranged such that the polarization direction of the laser beam is substantially parallel to the Nm axis of the first laser medium C1 and the second laser medium C2, .

That is, the first laser medium C1 and the second laser medium C2 are arranged such that the average cross-sectional area of the first laser medium C1 and the second laser medium C2 is increased to improve the average power of the beam. do.

At this time, the first laser medium C1 and the second laser medium C2 are arranged such that the polarization direction Epump of the pumping light is substantially parallel to the Nm-axis of the first laser medium C1 and the second laser medium C2, respectively . Accordingly, the absorption cross section for absorbing the pumping light generated from the laser diode into the first laser medium (C1) and the second laser medium (C2) is increased, and the laser beam generating efficiency is improved.

The femtosecond laser apparatus according to the embodiment of the present invention based on FIG. 10 can use three or more laser media in the embodiment of FIG.

11 is an optical conceptual diagram of another femtosecond laser device for explaining a layout structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to an embodiment of the present invention.

A schematic conceptual view according to the femtosecond laser device of FIG. 11 is similar to the optical conceptual view of FIG. Therefore, description of the same features will be omitted.

As shown, the optical component 10 is disposed between the first laser medium C1 and the second laser medium C2. The optical component 10 is configured such that the polarization direction of the laser beam generated from the first and second laser media C1 and C2 is the Np axis of the first laser medium C1 and the Nm of the second laser medium C2 Axis so as to be substantially parallel to the axis.

That is, the optical component 10 can change the polarization direction of the laser beam substantially parallel to the specific optical axis of the laser medium, so as not to change the spectral band width of the laser beam pulse as well as to distort the beam according to the thermal effect appearing in the laser medium. do. Therefore, the laser beam generated from the laser medium can be made closer to the circular shape as shown in FIG. 9 (c), thereby improving the quality of the beam.

At this time, the optical component 10 can be any component that can change the polarization direction of the beam, such as a polarization converter. The polarization converter may be a half-wave plate, a plurality of Fresnel rhombs, a broadband prismatic rotator, a Faraday rotator, or a combination of a plurality of mirrors .

The description of the laser beam generating efficiency, the output compensation, the number of laser media, and the like are the same as those described in the optical conceptual diagram of FIG. 8, and a description thereof will be omitted.

On the other hand, as in the embodiment of Fig. 10, the average output of the beam can be improved by adjusting the polarization direction of the laser beam using the optical component.

12 is an optical conceptual diagram of another femtosecond laser apparatus for explaining an arrangement structure of a laser medium included in a laser oscillator of a master oscillator or an amplifier according to an embodiment of the present invention.

The optical conceptual diagram according to the femtosecond laser apparatus of FIG. 12 is similar to the optical conceptual diagram of FIG. Therefore, description of the same features will be omitted.

As shown, the optical component 10 is disposed between the first laser medium C1 and the second laser medium C2. The optical component 10 is arranged such that the polarization direction of the laser beam generated from the first and second laser media C1 and C2 is the Nm axis of the first laser medium C1 and the Nm axis of the second laser medium C2 Axis so as to be substantially parallel to the axis.

That is, the optical component 10 makes the polarization direction of the laser beam substantially parallel to the specific optical axis of the laser medium, so as to increase the emission cross-sectional area of the laser medium to improve the average power of the beam.

At this time, the types and other features of the optical component 10 are the same as those of the embodiment of FIG. 11, and a description thereof will be omitted.

FIG. 13 is a view for explaining a laser beam generator included in a master oscillator or an amplifier according to an embodiment of the present invention, and FIG. 14 is a view for explaining a beam damper structure according to an embodiment of the present invention. 14 (a) is a plan view of a beam damper and a partial sectional view thereof, and FIG. 14 (b) is a front view of a beam damper and a partial sectional view thereof.

13 and 14, the laser beam generator 200 includes a first laser medium C1 and a second laser medium C2 disposed opposite to each other, and a second laser medium C2 disposed between the first laser medium and the second laser medium (30).

The beam damper 30 includes a hole 33 formed in the center of the beam damper 30 and a beam absorber 32 around the hole 33.

The hole 33 of the beam damper allows the laser beam generated from the first and second laser media C1 to pass therethrough and the beam absorber 32 of the beam damper passes through the first and second laser media C2 And can block or absorb pump beams that are not absorbed.

The pump beam generated by the laser diode (not shown) is absorbed by the first and second laser media C1 and C2, but the pumping light which is not absorbed by the laser is absorbed by the optical component (not shown) , A mount (not shown) supporting the optical component, and the like. Therefore, the laser device is thermally deformed as a whole, and the alignment characteristic of the beam is lowered.

The beam damper 30 according to the embodiment of the present invention may be configured such that the pumping light that is not absorbed by the first laser medium C1 or the second laser medium C2 can not be heated by the first laser medium C1) or the second laser medium (C2). Thereby preventing the alignment characteristic of the laser beam from deteriorating.

In addition, the beam damper 30 may be provided with a cooling water path 31 inside the beam damper 30 to prevent heat from being transmitted to the surroundings while being heated by the absorbed pumping light. The cooling water path 31 is supplied with cooling water so that the beam damper 30 can be sufficiently cooled. In addition, the cooling water passage 31 can form a path of the cooling water so as to be as close as possible to the beam absorbing portion 32 so as to increase the cooling efficiency.

An upper portion of the beam damper 30 may be formed with an insertion hole 35 into which the coupling means is inserted to be integrated with the optical component or optical mount constituting the laser generating device.

Such a beam damper 30 can prevent thermal deformation of the optical components generated in the process of generating or amplifying the femtosecond pulse when mechanically connected with other optical components. That is, the heat of the optical components by the pumping light can be transmitted to the beam damper through the connecting means and absorbed to prevent thermal deformation of the optical components.

Further, the beam damper 30 may have a screw coupling hole 36 at an upper end thereof so as to be fixed to the connecting means in cooperation with an optical mount (not shown).

The cooling water path 31 may be formed around the hole 33 or around the beam absorbing portion 32 to increase the cooling efficiency.

The beam absorbing portion 32 has a shape in which the diameter decreases from both ends of the beam damper 30 toward the central portion. That is, the beam absorbing portion 32 may have a conical shape in cross section so that the pumping light generated in the laser diode is incident on the beam damper 30 and is not reflected. Of course, the beam absorbing portion 32 can be any structure that can reduce the reflectance of the pumping light in the beam damper 30. [

In addition, the beam absorbing portion 32 may be anodized or coated with a material having a high light absorption rate to increase the absorption rate of the pumping light.

The following is an experimental example using the femtosecond laser device according to the present invention and the femtosecond laser system including the femtosecond laser device.

Experiment Example

A femtosecond master oscillator 110 was fabricated to generate a seed pulse to be applied to the amplifier 170.

At this time, the master oscillator 110 is the size 3 × 3 × 2mm ³ , Yb doping ratio 5at. % Yb: KYW laser medium.

The polarization direction of the pumping light is substantially parallel to the Nm axis of the laser medium and the oscillation polarization direction of the laser is substantially parallel to the Nm axis.

An Np-cut in which the laser medium was cut in the Np-axis direction was used to match the central wavelength of the oscillating femtosecond master oscillator 110 with the center wavelength of the resonator.

The center wavelength of the laser beam output from the master oscillator 110 was 1035 nm, the spectrum band width was 9.0 nm, the pulse time width was 110 fs, and the average output was 1.2 W.

The pulse expander 131 is a device for making the length of the pulse longer than the amplifier 170 in terms of time, and the pulse compressor 132 is a device for shortening the time width of the elongated pulse back to the femtosecond region.

In the chirped pulse amplification process, when a pulse of a time width that is elongated several hundreds or thousands of times is amplified by the amplifier 170, the peak power of the amplified pulse is lowered, Physical damage can be prevented.

In addition, it is possible to prevent the temporal shape of the pulse and the spatial distribution of the beam from being distorted by non-linear phenomena such as a self-focusing effect occurring at a high peak power.

In this experiment, one transmission diffraction grating with a groove density of 1500 lines / mm was designed and fabricated to serve as both the pulse expander 131 and the pulse compressor 132.

Test results of the pulse expander 131 and the compressor 132 using the femtosecond pulse of the master oscillator 110 are as follows.

The femtosecond pulse having a time width of 110 fs was expanded by a pulse expander 131 to a pulse of about 50 ps and then passed through the pulse compressor 132 to be compressed to 160 fs.

That is, the compression ratio indicating the ratio of the pulse time width before expansion to the pulse time width after compression was 1.45. The output conversion efficiency before and after the pulse expander 131 was 74%, and the output conversion efficiency before and after the pulse compressor 132 was 78%.

The pulses drawn out of the amplifier pass through only the desired pulses through the pulse picker 160 (pulse picker).

At this time, the pulse picker 160 separates the main pulse from the previous pulses (pre-pulses) and the subsequent pulses (post-pulses).

On the other hand, in the amplifier 170, two thin film polarizers are used to increase the contrast ratio between the laser pulse to be drawn out of the resonator by the Pockels cell 23 and the weak laser pulse which leaks out of the resonator .

Further, in the whole system, a thin film polarizer was additionally disposed to increase the final control ratio.

The amplifier 170 has a size of 2 × 2 × 5 mm ³ , a doping concentration of Yb ³ + ions of ^{3 at} . % Yb: KYW laser medium was used, and both ends of the laser medium were coated with anti-reflective coating against the pumping light source and the laser oscillation wavelength.

The dichroic flat mirror DM coated the pumping light in the 981 nm wavelength region with high transmittance and the reflectance with a high reflectance for the laser in the 1 micrometer wavelength region.

As the pumping light source, a high-brightness laser diode 21 having a wavelength of 981 nm and a maximum power of 70 W was used for each of the C1 and C2 laser media, that is, two laser light sources were used.

Yb: KYW used in this experiment exhibits different characteristics depending on the optical axis direction as anisotropic laser crystal.

Thus, the length of the optical fiber 22 fastened to the laser diode 21 is made as short as possible, so that the polarization direction of the pumping light is maintained to the maximum.

In this experiment, a high-brightness laser diode (21) having a length of 30 cm, a core diameter of 200 μm and a numerical aperture (NA) of 0.22 was used.

In order to maximize the pumping light to be absorbed into the laser crystal, the half wave plate? / 2 is disposed after the optical fiber 22 to finely adjust the polarization direction so that the polarization of the pumping light is substantially parallel to the Nm axis of the Yb: KYW crystal Respectively.

The laser medium C1 is arranged so that the polarization direction of the pumping light is substantially horizontal to the Nm-axis, while the polarization direction of the laser beam is substantially parallel to the Np-axis.

The laser medium C2 is an Np-cut cut in the Np-axis direction, the polarization direction of the pumping light is substantially parallel to the Nm-axis, and the polarization direction of the laser beam is also substantially parallel to the Nm-axis.

In other words, the polarization direction of the pumping light is substantially parallel to the Nm-axis with the largest absorption cross-section, and the polarization of the laser beam is substantially parallel to the Nm-axis with the largest radiation cross-section, Axis and substantially parallel to the large Np-axis to combine the different gain spectra.

A combination of laser media, one of which is Ng-cut and the other of which is Np-cut, in which the laser oscillation combines different gain spectral distributions to widen their band widths, resulting in shorter femtosecond pulses and lasers with different thermal characteristics. The polarization direction of the beam proceeds to disperse the thermal effect.

The length of the laser medium is 5mm and the doping rate is 3at. % Is to reduce the thermal lens effect and improve the spatial quality of the output beam at high power.

The LASCAD software (Las-CAD GmbH), which can simulate the output characteristics of a resonator by virtually simulating the output characteristics, can be used, for example, with a length of 5 mm and a doping ratio of 3 at. The Yb: KYW laser crystal receives a thermal lens and the optical intensity of thermal-mechanical stress is 3mm in length, the doping rate is 5at. % Of the laser crystals were 1.5 times weaker than the ones received.

Computer calculations also showed that the astigmatism intensities of the thermal lens in the Ng-cut and Np-cut crystals are similar but the axes are different.

When the output of the pumping light applied to the laser medium was 36 W, the ratio (fx / fy) of the thermal focal distance in the x-axis direction and the y-axis direction was 1.15 at Ng-cut and 0.88 at Np-cut.

This is because the ratio of the beam to be amplified when the laser beam passes through the Np-cut crystal successively after passing through the Ng-cut crystal, or when the laser beam passes the Ng-cut crystal continuously after passing through the Np- It means that the difference in the scores can be partially canceled each other.

Although the doping concentration of Yb is applied as a specific value in the experimental example according to the embodiment of the present invention, the doping concentration of Yb is in the range of 1 to 10 at. % Can be applied.

The present invention relates to a femtosecond laser device and an experimental result of a femtosecond laser system including the femtosecond laser device according to an embodiment of the present invention.

Experiment result

15 is a graph showing the relationship between the continuous wave (CW) output power (CW) according to the pumping power (incident pump power on crystals [W]) applied to the laser crystal in the continuous oscillation mode ).

First, when pumping light was applied to only one laser medium having Ng-cut and Np-cut, the slope efficiency was 47% and 37%, respectively.

At this time, when the pumping light applied was 36W, the maximum output was 12W and 9W, respectively.

When pumping light was simultaneously applied to laser media having Ng-cut and Np-cut, the slope efficiency was about 35%, and when the applied pumping light was 72W, the continuous wave output of the amplifier 170 was 18W.

In the Q-switch mode, an average output of 16W was obtained when the gate time was 800 ns and the pulse repetition rate was 200 kHz.

The reason why the output is slightly lower than the continuous wave mode is the loss due to the optical switch disposed inside the resonator.

The pulse time width was about 20 ns and the spectral bandwidth of the pulse was about 16 nm.

The pulse spectrum showed an M-shape with two peaks at 1035 nm and 1043 nm depending on the different gain peaks of the two laser media.

A spectrum narrowing occurs in the process of amplifying the pulse energy in the amplifier 170. FIG. Spectral shaping can be performed in an extra-cavity or intra-cavity using a polarization-interference filter called a Lyot filter to suppress this and broaden the spectral bandwidth. -cavity).

On the other hand, the spectral shaper 140 is composed of two polarizing plates 122 and a birefringent quartz plate 121 disposed therebetween as shown in FIG.

For optimal spectral shaping, the minimum point of transmission by the birefringence filter should be equal to the maximum point of the gain spectrum, and the widths should be similar to each other.

In order to realize this, a quartz plate having a thickness of 8 mm is cut along the optical axis and mounted on the rotary mount 123 so as to be able to rotate precisely in the rotation direction? And? Direction, so that the position of the minimum transmission point and the modulation depth thereof are finely Respectively.

As a final step, pulse extension and spectrally shaped seed pulses are applied to the amplifier 170 to configure the overall system as shown in FIG. 1 and to measure its operating characteristics.

FIG. 17 shows spectra of the pulses chirped by the pulse expander 131 (a) before and after (b), respectively.

The pulse from the master oscillator 110 of Figure 1 shows a symmetric spectrum with a center wavelength of 1035 nm and a bandwidth of 9 nm.

It can be seen that after spectral shaping, it has a locally maximum value near 1030 nm and 1040 nm.

Of course, spectral shaping can be performed in various forms by adjusting the thickness, rotational direction?, And? Of the quartz plate constituting the spectrum molding machine 140.

18 (a) shows a case where a seed pulse not subjected to spectral shaping (SS) is applied to the amplifier 170 and pumping light of the same intensity is applied to the C1 and C2 laser crystals at a pulse repetition rate of 200 kHz, And shows an asymmetric spectrum with a center wavelength of 1036 nm and a bandwidth of 6 nm.

Compared with Fig. 17 (a), which is a pulse spectrum before amplification, it can be clearly seen that the narrowing of the gain narrows the spectral band width from 9 nm to 6 nm.

When the pulse of this narrowed spectrum is compressed by the pulse compressor 132, the pulse time width is measured as 265 fs as shown in Fig. 19 (a).

This narrowing of the spectrum can be suppressed in various ways.

For example, when pumping light of different intensities is applied to C1 and C2 of anisotropic laser crystal Yb: KYW with a laser medium, gain spectra of different intensities are combined because they have maximum gain values at different center wavelengths Can be obtained.

In the actual experiment, it was confirmed that when the ratio of the pumping power applied to the Np-cut crystal and the Ng-cut laser medium is changed to 3: 2, the spectrum becomes wider and the shape is significantly deformed.

In this case, the spectral band width was 9 nm and the pulse time width was 210 fs. However, in this case, although a narrow pulse width can be obtained, there is a disadvantage that the total output of the femtosecond laser system is limited by limiting the intensity of the pumping power.

In this experiment, the output was measured to be reduced by 37% under the above conditions.

Another way to suppress spectral narrowing is to use spectral shaping. As shown in FIG. 1, a spectral shaping unit 140 as shown in FIG. 16 is disposed between a pulse expander 131 and an amplifier 170 to perform spectral shaping of a seed pulse from the outside of the resonator.

The optimum angle is determined while observing the spectrum of the pulse amplified and the time width of the pulse while finely adjusting the angle of the rotation mount 123, phi, and phi.

18 (b) shows the spectrum of the amplified laser pulse when the seed pulse is spectrally shaped.

The measured spectrum was a bell-shape with a center wavelength of 1034 nm and a spectral bandwidth of 11 nm.

Before spectral shaping, the band width of the amplified pulse was 6 nm, and after spectral shaping, the band width was almost doubled to 11 nm. When the pulse of the broadened spectrum is compressed by the pulse compressor 132, the pulse time width was measured as 182fs as shown in FIG.

Experiments were performed by placing the spectral shaping machine 140 inside the resonator of the amplifier 170.

The spectrum of almost the same width was obtained, but the laser output was reduced by about 20%.

This is because a small loss is accumulated by the spectrum forming machine 140 including the reed filter while the laser pulse reciprocates several times inside the resonator.

20 shows a change in pulse energy with respect to the pulse repetition rate.

The lower the pulse repetition rate, the higher the pulse energy. The maximum pulse energy was measured as 164 μJ when the pumping power applied to the laser crystal was 73.2 W and the pulse repetition rate was 50 kHz.

Experimental observations have shown that Raman scattering occurs at low pulse repetition rates, which acts as an impediment to further increasing the pulse energy.

Pulse energy of 10 ~ 50μJ was obtained at a pulse repetition rate of 200 kHz and pulse energy of 4 ~ 20μJ was observed at 500 kHz.

This level of pulse energy is sufficient to process various samples at high pulse repetition rates.

The femtosecond laser device according to an embodiment of the present invention and the femtosecond laser system including the femtosecond laser device are not limited to the embodiments described above but are defined by the claims, It is obvious that various modifications and alterations can be made within the scope of the right

110: master oscillator 131: pulse expander
140: spectrum shaping machine 170: amplifier
150: Parabody rotator 160: Pulse picker
120: Paradieter isolator 132: Pulse compressor
21: laser diode 22: optical fiber
23: Pockels cell 30: beam dumper
31: cooling water passage 32: beam absorbing portion

Claims (17)

A first laser medium having spatially mutually perpendicular Ng, Np and Nm axes; An Np axis substantially parallel to the Ng axis of the first laser medium, an Nm axis substantially parallel to the Np axis of the first laser medium, and an Ng axis substantially parallel to the Nm axis of the first laser medium A second laser medium; And a first laser diode and a second laser diode arranged to irradiate pumping light to each of the first laser medium and the second laser medium, the femtosecond laser device comprising:
Wherein the first laser medium comprises:
Wherein a traveling direction of the laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium,
The polarization direction of the laser beam generated from the first and second laser media is arranged to be substantially parallel to the Np axis of the first laser medium,
Wherein the second laser medium comprises:
The advancing direction of the laser beams generated from the first and second laser media is substantially parallel to the Np axis of the second laser media,
And a polarization direction of the laser beams generated from the first and second laser media is substantially parallel to the Nm axis of the second laser media.
The method of claim 1,
The first and second laser media are at least one of Yb: KYW, Yb: KGW, Yb: KLuW, Yb: YCOB, and Yb: YAP, respectively.
3. The method of claim 2,
The doping concentration of Yb in the Yb: KYW, Yb: KGW, Yb: KLuW, Yb: YCOB and Yb: YAP is 1 to 10 at. Femtosecond laser device, characterized in that%.
The method of claim 1,
Wherein the first laser medium comprises:
The polarization direction of the first pumping light generated from the first laser diode is substantially parallel to the Nm axis of the first laser medium,
Wherein the second laser medium comprises:
And a polarization direction of the second pumping light generated from the second laser diode is substantially parallel to the Nm axis of the second laser medium.
A first laser medium having spatially mutually perpendicular Ng, Np and Nm axes; An Np axis substantially parallel to the Ng axis of the first laser medium, an Ng axis substantially parallel to the Np axis of the first laser medium, and an Nm axis substantially parallel to the Nm axis of the first laser medium A second laser medium; And a first laser diode and a second laser diode arranged to irradiate pumping light to each of the first laser medium and the second laser medium, the femtosecond laser device comprising:
Wherein the first laser medium comprises:
Wherein a traveling direction of the laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium,
The polarization direction of the laser beam generated from the first and second laser media is arranged to be substantially parallel to the Nm axis of the first laser medium,
Wherein the second laser medium comprises:
The advancing direction of the laser beams generated from the first and second laser media is substantially parallel to the Np axis of the second laser media,
And a polarization direction of the laser beams generated from the first and second laser media is substantially parallel to the Nm axis of the second laser media.
A master oscillator for generating pulses in a femtosecond range;
A pulse expander for expanding the pulse width of the generated femtosecond region;
An amplifier for amplifying the energy of the extended pulse; And
And a pulse compressor for compressing the amplified pulse into pulses in a femtosecond range,
At least one of the master oscillator and the amplifier comprises the laser device of claim 1.
The method according to claim 6,
Wherein the pulse expander and the pulse compressor are integrally formed by using one spectroscopic element in common.
The method according to claim 6,
And a spectral shaper for shaping the spectrum of the extended pulse.
A first laser medium and a second laser medium having Ng, Np and Nm axes spatially perpendicular to each other and arranged to face each other;
A first laser diode and a second laser diode arranged to respectively inject pumping light into the first laser medium and the second laser medium;
Disposed between the first laser medium and the second laser medium, wherein the polarization directions of the laser beams generated from the first and second laser mediums are on the Np axis of the first laser medium and the Nm axis of the second laser medium; Includes an optical component to be substantially parallel,
The first laser medium
Wherein a traveling direction of the laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium,
Wherein the second laser medium comprises:
The femtosecond laser device, wherein the direction of travel of the laser beams generated from the first and second laser media is substantially parallel to the Np axis of the second laser media.
The method of claim 9,
The optical component may include at least one of a half-wave plate, a plurality of Fresnel rhombs, a broadband prismatic rotator, a Faraday rotator, and a plurality of mirrors. Femtosecond laser device, characterized in that one.
The method of claim 9,
The first and second laser media are at least one of Yb: KYW, Yb: KGW, Yb: KLuW, Yb: YCOB, and Yb: YAP, respectively.
12. The method of claim 11,
The doping concentration of Yb in the Yb: KYW, Yb: KGW, Yb: KLuW, Yb: YCOB and Yb: YAP is 1 to 10 at. Femtosecond laser device, characterized in that%.
The method of claim 9,
Wherein the first laser medium comprises:
The polarization direction of the first pumping light generated from the first laser diode is substantially parallel to the Nm axis of the first laser medium,
Wherein the second laser medium comprises:
And a polarization direction of the second pumping light generated from the second laser diode is substantially parallel to the Nm axis of the second laser medium.
A first laser medium and a second laser medium having Ng, Np and Nm axes spatially perpendicular to each other and arranged to face each other;
A first laser diode and a second laser diode arranged to respectively inject pumping light into the first laser medium and the second laser medium;
Disposed between the first laser medium and the second laser medium, wherein the polarization directions of the laser beams generated from the first and second laser media are on the Nm axis of the first laser medium and the Nm axis of the second laser medium; Includes an optical component to be substantially parallel,
The first laser medium
Wherein a traveling direction of the laser beam generated from the first and second laser media is substantially parallel to an Ng axis of the first laser medium,
Wherein the second laser medium comprises:
The femtosecond laser device, wherein the direction of travel of the laser beams generated from the first and second laser media is substantially parallel to the Np axis of the second laser media.
A master oscillator for generating pulses in a femtosecond range;
A pulse expander for expanding the pulse width of the generated femtosecond region;
An amplifier for amplifying the energy of the extended pulse; And
And a pulse compressor for compressing the amplified pulse into pulses in a femtosecond range,
At least one of the master oscillator and the amplifier comprises the laser device of claim 9.
16. The method of claim 15,
Wherein the pulse expander and the pulse compressor are integrally formed by using one spectroscopic element in common.
16. The method of claim 15,
And a spectral shaper for shaping the spectrum of the extended pulse.

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